The TZE number on A GMC is on a plate on the passenger side firewall, if the front hatches are unlocked you can see it, it's also stamped on the frame rail but after that many years the rust makes them hard to make out and you can't get under them to see if it's been sitting a long time, once you find the number you can ask for prior history if they have it here,
www.gmcmhregistry.com